Polylactic acid synthesis, properties and technical and biomedica applications

MAIN SYNTHESIS METHODS OF POLYLACTIC ACID On an industrial scale, manufacturers have developed some synthesis methods for high molecular weight PLA Mw> 10,000 g/mol.. PLA obtained in thi

POLYLACTIC ACID: SYNTHESIS, PROPERTIES AND

TECHNICAL AND BIOMEDICAL APPLICATIONS

Nguyen Thuy C hinh1’2, Thai H oang1,2’*

1 Graduate University o f Science and Technology, Vietnam Academy o f Science and Technology,

18 Hoang Quoc Viet, Cau Giay, Ha Noi, Viet Nam 2Institute fo r Tropical Technology, Vietnam Academy o f Science and Technology,

18, Hoang Quoc Viet, Cau Giay, Ha Noi, Viet Nam

*Email: hoangth@itt.vast.vn

Received: 9 November 2021; Accepted for publication: 20 Junuary 2022

Abstract Polylactic acid (PLA) is one of the common aliphatic polyesters synthesized from

lactic acid monomer (2-hydroxyl propionic acid) by fermentation or chemical synthesis Due to its high strength, high modulus, biodegradability, compostability and well-known safety profile, PLA becomes a very useful material for both fundamental researches and practical applications However, awareness of PLA manufacturing knowledge combined with understanding of its physico-chemical properties is essential for fruitful applications of PLA This review article presents the synthesis, characteristics, properties and applications in technique and biomedicine fields of PLA Among them, main synthesis methods of PLA will be mentioned The physical, mechanical, thermal, gas permeable, electrical, and chemical properties of PLA will be described The applications of PLA in packaging materials, agriculture, or other technique fields and biomedicine also help readers have a better overview of PLA

Keywords: polylactic acid, properties, degradation, application, biomedicine.

Classification numbers: 1.4.2, 2.9.3.

1 INTRODUCTION

Polylactic acid or polylactide (PLA), a biodegradable hydrolyzable aliphatic semi­crystalline polyester having no aromatic structure, was discovered in the 1920s by Wallace Corothers It is produced from renewable agricultural sources such as com, rice, wheat, sugar beets, and other starchy products, thus it is known for its eco-friendliness In general, PLA is produced through the direct condensation reaction of its monomer, lactic acid, as an oligomer, and followed by a ring-opening polymerization of the cyclic lactide dimer Its chemical structure can be seen in figure 1 Its properties vary to a large extent depending on the ratio between, and the distribution of two stereoisomers or other co-monomers For industrial applications (films,

Nguyen Thuy Chinh, Thai Hoang

(PDLLA) (Figure 2) PLLA has high crystallinity and slow degradation rate while PDLA can be decomposed more quickly Both low molecular weight PLA and high molecular weight PLA have been synthesized, however, low molecular weight PLA is less stable, so this synthesis was considered unsuccessful [2] Therefore, high molecular weight PLA is mainly produced in industry and widely applied in various fields of life and industry

Figure 2 (a) The stereoisomers of lactic acid and (b) Chemical structure of PLLA (bl),

PDLA (b2) and PDLLA (b3)

PLA is the one of most important bioplastics in terms of consumption volume in the world Some largest PLA producers are NatureWorks (Joint-venture between Cargill (US) and PTT (Thailand)), WeforYou, Evonik, Total-Corbion (Joint-Venture between Total (France) and Corbion (Netherlands)) The production and use of PLA reduce greenhouse gas emissions and environmental impact compared to alternative polymers including polycarbonate, polystyrene, polyethylene terephthalate, polypropylene, low density polyethylene [3] PLA can be processed

at 170-230 °C by injection molding, sheet blow molding, thermoforming, sheet extrusion, fiber spinning, injection stretch blow molding or non-woven spinning, and spun bonding [1] It is

primarily hydrophobic because of the presence of methyl groups of the LA monomers It can be completely degraded into carbon dioxide and water by microbes It has good biocompatibility, hence, it has been approved by the U.S Food and Drug Administration (FDA) and European regulatory authorities for use in drug-delivery systems and food.

2 MAIN SYNTHESIS METHODS OF POLYLACTIC ACID

On an industrial scale, manufacturers have developed some synthesis methods for high molecular weight PLA (Mw> 10,000 g/mol) These are lactide ring opening polymerization (ROP), structural co-monomers in high boiling solvents/direct polymerization, and chain extension (Scheme 1) The majority of commercial producers find that ROP is preferable for better process control and better production quality Therefore, most of PLA products on the market have been produced according to ROP route Only minor amounts of PLA have come from other routes

ROP route includes a three-step reaction: polycondensation, lactide formation and lactide ring-opening polymerization In the first stage lactide is formed, which - after fine purification -

is converted by ROP to PLA Firstly, lactic acid was evaporated or distilled to remove water and concentrate lactic acid, followed by pre-condensation to form pre-polymer Next, the pre­polymer converts to a cyclic dimer (crude lactide) Then, the lactide was purified to obtain highly purified lactide before it was ring opening polymerized to form PLA After that, the demonomerisation or stabilization was carried out to obtain high purity polylactide [1] PLA obtained in this way has high molecular weight, higher than 100,000 Daltons [4],

PLA production

Polycondensation Ring-opening polymerization Chain extension

Scheme 1 Synthesis scheme of PLA by different methods.

Lactide can be polymerized in melt, bulk, solution or suspension state Some parameters that can affect PLA processing through ROP route are racemization, lactide purity and residual monomer content The metallic catalysts which have been typically used in ROP to produce PLA include tin-based catalysts, aluminum alkoxides, etc due to their solubility in molten lactide, low rate of racemization of the polymer and high catalytic activity [1, 2] The above efficient catalysts based on Ca, Fe, Mg, Zn and K showing less toxicity than tin compounds have been used in lactide and lactone polymerization In addition, metal-free catalysis including organocatalytic (cationic, nucleophilic, bifunctional) or enzymatic approaches has been

Nguyen Thuy Chinh, Thai Hoang

developed for ROP to form PLA Depending on the catalyst, the ROP o f lactide can occur according to one of three mechanisms: anion, cation and coordination/insertion Among the above catalysts, tin (II) di-2-ethyl hexanoic acid (typically tin (II) octoate or stannous octoate) was approved by the FDA because it has high catalytic activity, low toxicity and is a highly suitable inducer The advantage of this route is that PLA has a high molecular weight, however,

it requires strict purity of the LA monomer and has high cost

The direct polymerization of LA monomers is also used to produce PLA This way can be carried out in bulk followed by chain extension with reactive additives or by solid state post­condensation to reach high molecular weight PLA The direct polycondensation can be divided into solution polycondensation method (using solvents) and melt polycondensation method (non-solvent) The advantages of this route are one-step, ease to control and economical cost However, the disadvantages of solution polycondensation method are impurities, side reactions, pollution, and low molecular weight PLA, while the limitations of melt polycondensation method are high reaction temperature, sensitivity to reaction conditions, and low molecular weight PLA The melt condensation polymerization includes three stages: removal of free water, oligomer polycondensation and melt polycondensation of high molecular weight PLA In the first stage, hydroxyl and carboxylic acid groups of LA monomers react together and water has been removed during the condensation reaction (Eq 1)

where n and m > 1

During condensation process, the ring structure such as lactide can be formed This can make a negative effect on the properties of obtained PLA The temperature of reaction should be below 200 °C to avoid the formation of lactide However, this causes an effect on the removal of water

In the second stage, the low molecular weight PLA or lactide oligomers which converted from LA was obtained Strong acidic organometallic compounds based catalysts are used to improve the reaction rate in this stage

In the third stage, the removal of water reaches critical The melt polycondensation should

be carried out to enhance both mass and heat transfer of water The reactive additives have been added to chain extension of PLA After melt polycondensation, the melt polycondensated PLA was cooled and particles were formed These particles can be subjected to a crystallization process The PLA obtained by this route has molecular weight as high as 130,000 g/mol or 100,000 g/mol or 63,000 to 79,000 g/mol depending on the type of catalyst used [2], In some cases, drying organic agents were used in the azeotropic dehydration reaction, the PLA can retain the optical purity However, these solvents are flammable causing safety risks Besides, the chain extenders and polymer impurities are toxic and non-biodegradable, thus, the obtained PLA by this route cannot apply in biomedicine field [4]

When using catalysts or adding dried organic agents during polycondensation process, the obtained PLA has a high molecular weight, the process is more efficient and has no pollution.Another route for PLA synthesis is chain extension using linking agents such as diisocyanates, bis-2-oxazoline, dual (2,2’-bis(2-oxazoline) and 1,6-hexam-ethylenediisocyanate), or bis-epoxies In the presence of linking agents, it is possible to control PLA branching [2] In addition, PLA can be biosynthesized in one-step The obtained PLA has high molecular weight and high degradable capacity

3 CHARACTERISTICS, PROPERTIES OF POLYLACTIC ACID

3.1 Physical properties

PLA is a white or opaque yellow polyester thermoplastic It has high gloss and transparency The specific weight of PLA is 1.25 g/cm3 The physical and other properties of PLA depend on two isomers of LA monomers, L-lactic and D-lactic (produced by the fermentation of carbohydrates by homofermentative bacteria and heterofermentative bacteria, respectively), with three optical isomers of LA [5] In addition, lactide purity also affects the properties of PLA The solubility of PLA depends on the molecular weight, crystallinity and content of co-monomers in the polymer PDLA, PLLA and PDLLA have crystal, hemi-crystal and amorphous structures, respectively [6] PLA can be dissolved in fluoride or chloride organic solvents, dioxane, furan, acetone, pyridine, ethyl lactate, tetrahydrofuran, xylene, ethyl acetate, dimethylsulfoxide, N,N-dimethylformamide, and methyl ethyl ketone It is insoluble in water, alcohols (methanol, ethanol, propylene glycol), and unsubstituted hydrocarbons (hexane, heptane) [7], The specific viscosity or intrinsic viscosity of PLA can be determined in chloroform or benzene using a Ubbelohde viscometer at 30 °C The molecular weight of PLA could be calculated according to the Mark-Houwink equation [8-10]:

where, K and a are constants, depending on nature of solvent and temperature, the value of a

ranges from 0.5 to 0.8, M is molecular weight, and [rj] is intrinsic viscosity (in dL/g)

The polydispersity of isotactic PLA (S-PLA) and racemic/atactic PLA (r-PLA) given in

Table 1 was calculated using the Mark-Houwink equation and reported by Schindler et al [8].

Table 1 Mark-Houwink parameters and polydispersity of isotactic PLA (S-PLA) and

racemic/atactic PLA (r-PLA) in chloroform and benzene [8]

as mentioned above PLLA has Young modulus of 2.7 GPa while PDLA has Young modulus of 1.9 GPa [11] As the molecular weight increases, the mechanical properties of PLA are enhanced For instance, when the molecular weight of PLLA increases from 23,000 to 67,000 g/mol, its flexural strength increases from 64 to 106 MPa and the tensile strength reaches 59

MPa For PDLLA, when its molecular weight increases from 47,500 Id 11 4 j M | W t i i l n B k

strength and torsional force vary from 49 MPa to 53 MPa and from 84 M h tn V MPa respectively [6] Since PLLA has a semi-crystalline structure, it has better modnuimi pmpataes

than PDLLA which is in amorphous state The conversion process can produce PLA m either

oriented or non-oriented form The orientation can affect the mechanical properties o f PLA Table 2 shows a clear difference in critical tensile strength, tensile strength, Young modulus, elongation at break and impact strength of oriented PLA as compared to non-oriented PLA This

is caused by the degree of orientation as well as the content of the - stereostructure o f die two PLAs above However, those factors do not affect the Rockwell hardness, density, and glass

transition temperature of these two PLA types [12]

Table 2 Mechanical properties of high molecular weight PLA with oriented and non-oriented

macromolecular chains [12]

Table 3 Effect of stereostructure and crystallinity on the mechanical properties of some PLA types

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and stereo-regulation of polymer macromolecular chains, the elongation and impact strength of PLLA have been improved significantly [13-14],

Table 4 Mechanical properties of PLA plasticized with PEG [15],

(MPa)

Elongation at break (%)

Young modulus (GPa)

PLA -Jr- LDPE PLA -A- LDPE

-O - PLA/PBAT -d r PVC - O PLA/PBAT - A - PVC

Figure 3 Tensile strength and elongation at break of PLA, LDPE, PVC, and PLA/PBAT blend [16]

(Reprinted from Katsuyoshi S by permission of Hindawi Limited)

As PLA is combined with another polymer, its mechanical properties of PLA can be enhanced For example, a blend of poly(glycolic acid) and PDLA (75/25 wt.%) has a Young modulus of 2.0 GPa, 0.1 higher than neat PDLA (1.9 GPa) [11],

To increase the elongation at break and elasticity of PLA, some low molecular weight and biodegradable compounds including lactite monomer, glucose monoesters, fatty acid partial esters, citrate esters (triacetin citrate, tributyl citrate), epoxidized soybean oil (ESO), and acetyl tri-n-butyl citrate (ATBC) as well as polymers such as polyethylene glycol (PEG) and polycaprolactone (PCL) are used to plasticize PLA [15] These above materials have low glass transition temperatures, thus, they can reduce the glass transition temperature of PLA when mixed, resulting in PLA becoming softer The elongation at break of PLA increased when plasticizers were introduced into PLA macromolecules, reducing the interaction and bonding of PLA chains For example, when mixing PLA with PEG (two PEG types with different molecular weight, PEG 400 and PEG 1,500), the tensile strength of PLA decreased rapidly from 66 MPa to 18.5- 18.7 MPa while its elongation at break increased by more than 100 times (Table 4) [15]

As a result, PLA becomes more flexible and it can be applied to packaging and film products

Nguyen Thuy Chinh, Thai Hoang

The mechanical properties of PLA depend on temperature When increasing the temperature from -20 °C to 40 °C, the tensile strength of PLA decreased while its elongation at break did not change (Figure 3) The tensile strength of PLA was higher than that of low density polyethylene (LDPE), poly(vinyl chloride) (PVC) or blend of PLA and polybutyrate adipate terephthalate (PBAT) [16]

3.3 Thermal properties

PLA has a higher melting temperature (Tm) and crystallization temperature (Tc) than LDPE and high-density polyethylene (FIDPE) It is difficult for PLA to degrade by heat or thermal - oxidation as compared to LDPE and HDPE At temperatures greater than the glass transition temperature (Tg), PLA changes from the glassy to the rubbery state When PLA is heated to a temperature greater than Tg, it gradually changes to a viscous form At temperatures below Tg, PLA is in the glassy state and is capable of stretching when cooled down to a second transition

or [3-transition temperature of about -45 °C Below this temperature, PLA is a rather brittle polymer The thermal properties of PLA are highly dependent on the stereostructure (Table 5) PLLA has a Tm of 170 - 183 °C and a T, of 55 -65 °C while PDLLA has a Tg of 59 °C The Tm of PLLA can increase from 40 - 50 °C and this temperature also increases to 60 - 190 °C when it has been blended with PDLA Melt enthalpy of 100 % crystalline PLA (AFI0m) is 93 J/g The Tm and the degree of crystallization of PLA depend on the molecular weight, purity, crystallization kinetics, etc [17], The thermal stability of PLA decreases rapidly under high temperature and humidity conditions The oriented PLA and non-oriented PLA have the same Tg value, around

57 - 60 °C [12], For PLA combined with poly(glycolic acid), the Tg of PLA decreased slightly (5

- 10 °C) [11] while for PLA blended with PEG, the Tg of PLA decreased sharply, from 60 °C to

22 °C [15],

Table 5 Thermal properties of some PLA types [13, 17].

Table 6 DSC parameters and XRD results of PLA samples [18],

(°C)

Due to its high moisture permeability index and low Tg, PLA is difficult to mold at high

temperatures compared to PE and PLA having low stability Carrasco et al studied the chemical

structure, degree of crystallinity, thermal stability and other properties of PLA after processing

on industrial plastic processing machines (injection and extrusion after post- injection) with or without annealing process (Table 6) [18]

The authors found that PLA processing caused a reduction in the molecular weight of PLA (determined by gel permeation chromatography) by breaking the PLA macromolecular chains The degree of crystallinity of PLA was determined by differential scanning calorimetry (DSC) and X - ray diffraction (XRD) methods The rapid cooling process of the PLA sample after injection molding hardly or less causes a re-arrangement in the crystalline structure of PLA In contrast, after annealing, a crystalline structure of PLA was formed By using the DSC method, the authors determined the degree of crystallinity of injection PLA (PLA - 1), extrusion after post

- injection (PLA - El), and annealed PLA (PLA - EIA) to be 4 %, 8 %, and 33-35 %, respectively By using the XRD method, the degree of crystallinity of annealed PLA was determined to be 45-47 % PLA had a higher thermal stability (~331 °C) than processed PLA (323-325 °C) [18]

Mohamed et al also reported the effect of annealing process on the thermal properties of

PLA Increasing PLA annealing time of PLA from 0 to 24 hours could lead to an increase in the degree of crystallization of PLA (Table 7) The PLA sample annealed for 24 hours had a thermal conductivity of 0.0904 W/(m.K) and a Tg of 59.0 °C, an increase of 40.59 % and 11.33 %, respectively, compared to the unannealed PLA An annealing time from 1 to 3 hours at 90 °C was suitable for PLA to apply as a green thermal insulation material (with thermal conductivity

3.4 Gas permeability

Nguyen Thuy Chinh, Thai Hoang

PLA has good gas permeability Table 7 presents the gas permeability of some common thermoplastics [21, 22] The gas permeability of PLA, especially for N2 and 0 2, is much lower than that of PE The C 0 2 permeability of PE is many times higher than that of PLA This means that PLA shields the air much better than PE In addition, PLA has good odor retention [21, 22]

The shielding ability and the gas permeability coefficient (including oxygen and C 0 2) of PLA are smaller than those of polystyrene (PS) but almost similar to those of PET (Table 9) It is noteworthy that the water-vapor permeability of PLA does not change significantly with relative humidity even though PLA is a polar polymer The decrease in water-vapor permeability of PLA with increasing temperature is an advantage of PLA for use as a multilayer structural encapsulation material [23]

Table 8 Gas permeability of PLA, HDPE, LDPE, and PET [21-22].

Water-vapor permeability coefficient [kg/m/(m2/s/Pa)]

Oxygen transmission rate [cc(m2/d)]

Oxygen permeability coefficient [kg/m/(m2/s/Pa)]

on the PLA sample, etc Table 10 gives dielectric constant values at very low frequencies (erfl), dielectric constants at very high frequencies (eroo), recovery intensity (As) and recovery time (x)

of the original PLA sample with 5% crystallinity, before heat treatment (denoted as PLA-0) and the PLA sample with 42 % crystallinity, after 15 minutes of heat treatment at 80 °C (denoted as PLA-A) It can be seen that the recovery intensity value of PLA-0 decreased significantly after heat treatment, from 2.15 to 0.88 This can be explained by the crystallization process after heat treatment which restricts/prevents the orientation of the dipoles In addition, the x value of the PLA-A sample increased very strongly, 7 times higher than that of the PLA-0 sample Thus, crvstallization in PLA after heat treatment contributed to the suppression of dipole orientation

[24].

292

Table 10 Dielectric constant, recovery intensity and recovery time of PLA before and after

Table 11 Dielectric constant and dielectric loss tan8 of PLA and ABS [25].

3.6 Chemical properties

PLA is a saturated polyester which is easily hydrolyzed by chemical agents and enzymes of microorganisms It is also easily decomposed under the influence of high temperature, sunlight radiation, etc In a humid environment, especially when in the presence of acids and bases, PLA

is hydrolyzed and the ester groups of its main chain were cleaved, leading to a decrease in the molecular weight of PLA In aqueous media, PLA degradation is mainly due to hydrolysis of ester bonds and occurs randomly along the polymer chain:

Nguyen Thuy Chinh, Thai Hoang

while the pH in the deeper layer inside the sample decreased because of the formation of end- chain acid groups of PLA [26] The dispersion ability of the oligomers into the buffer medium was difficult due to the sample thickness PLA oligomers containing the end-chain carboxyl groups can escape from the sample when the sample thickness was about 200-300 pm Samples with thickness thinner than 200 pm could degrade much more slowly due to the rapid escape of oligomers containing the end-chain carboxyl groups into the medium, reducing the autocatalytic effect [27] The self-catalyzed PLA hydrolysis reactions take place as follows:

In acidic medium, the hydrolysis of PLA has additional stages of protonation and proton separation The protonated ester groups make the carbon atom in the carbonyl bond becomes more active and susceptible to attack by the nucleophile (H20 ) [30], Then, acyl-oxygen cleavage forms lactic acid oligomfrs or PLA molecules containing terminal -COOH groups These groups

in turn catalyze the hydrolysis of PLA following the “self-catalysis” mechanism as described above [26, 31]

slow fast

When packaging films, bags, and boxes made from PLA contact with food, lactic acidosis may occur during storage time because PLA decomposes in high humidity condition The major compounds degraded from PLA are LA, lactide and oligomers The survey results show that PLA is stable at 40 °C for 6 months, decomposes at 60 °C or above Tg and the degree of lactic acidosis increases Regarding the toxicity of lactic acid, the Food and Agriculture Organization

of the United Nations (FAO)/World Health Organization (WHO/Expert Committee on Food Additives (JECFA)) states that both D-lactic acid and (DL)-lactic acid leached from PLA should not be used in infant food packaging [23]

PLA is susceptible to microbial effects The decomposition process of PLA by microorganisms consists of two stages In the initial stage, microorganisms secrete enzymes that catalyze the hydrolysis of PLA into short-chain fragments of a size small enough for the microorganisms to use as a source of nutrients limited size of PLLA oligomers that microorganisms can digest is 11,000) [32] In the later stage, microorganisms digest the above hydrolysis products to produce biomass, C 0 2 and H20 PLA is degraded by some bacteria, such

as amycolatopsis sp One of the most effective PLA-degrading enzymes is proteinase K.

Table 12 indicates the cleavage of PLLA macromolecules and PLLA hydrolysis mechanism under different pH and temperature conditions, with and without enzymes It is noteworthy that a random cleavage of the PLLA macromolecules occurred in all investigated conditions The enzymatic hydrolysis and degradation of PLA in the microbial media are also greatly influenced by the molecular weight and crystallinity of PLA [33], The smaller the molecular weight of PLA, the greater the rate of biodegradation As the crystal content of PLLA increased, the strength of PLA also increased PLA obtained by LA polymerization had a low molecular weight and contained many end-chain carboxyl and hydroxyl groups, so they can react with monomers or polymers containing terminal functional groups such as carboxyl, hydroxyl, amino, anhydride acid, etc As a result, the PLA circuit was extended, increasing the molecular weight of PLA

Table 12 Cleavage of PLLA macromolecules and hydrolysis mechanism of PLLA film under

K

Circuit cleavage predominates in free- terminal chains and these chains were compacted in the amorphous region

Surface

PDLA and PLLA are more resistant to saline than PDLLA The half-life of PDLA and

thermal decomposition of PLA occurs in the temperature range of 325 - 375 °C to form lower

molecular weight chain-cut products (oligomers), volatile organic compounds, and CO [34],

Nguyen Thuy Chinh, Thai Hoang

4 TECHNICAL APPLICATIONS OF POLYLACTIC ACID

4.1 Packaging materials

PLA is used in food packaging, including direct contact applications where it is classified

as safe (GRAS) The packaging films based on PLA is very effective for products such as fresh fruits, coffee, foods, confectionery, cold drinks, etc It is used as high-transparency bottles to replace PET bottles, as food and beverage containers, cups, spoons, overwrap, etc [23, 35-37], Table 13 presents the potential applications of PLA as a packaging material for various foods

Table 13 Potential applications of PLA as a packaging material [23, 35 - 37],

Bottled soft drinks Shielding gases, moisture and light resistance, inert

with fragrance contamination

Noble™

Yogurt Mechanical strength, shielding oxygen, C 02, moisture

and fat resistance

Dannon™Cheese Shielding gases, moisture and light resistance

Butter/Margarine Mechanical strength, shielding gases, and moisture

resistance

PLA is also known as a green food packaging material because it can keep the smell and freshness of food after processing, including processing at high pressures (350, 450, 650 MPa for 15 minutes) Some modifiers can be added to PLA to improve its mechanical properties, gas and UV shielding properties, anti-microbial ability, and anti-oxidation ability, especially, modified PLA in the form of nanocomposites [35], PLA blends with starch, protein and biopolymers have been studied and developed for biodegradable and renewable packaging materials Packaging materials based on a combination of PLA with organic acids, antibiotics made from bacteria - bacteriocins (e.g nisin), extracts from fruits such as lemon and grapefruit, essential oils, enzymes (e.g lysozyme), and metals (e.g silver zeolites) are potential

antimicrobial packaging materials They showed inhibitory effects on Listeria monocytogenes,

Escherichia coli, Staphylococcus aureus, and Micrococcus lysodeikticus Native antibacterial

substances were also coated on the surface of antibacterial packaging products from PLA [38],Recently, Purac Company (Netherlands) has developed heat-stable PLA fabrication technology to make products such as cups, bottles, and jars for hot foods and beverages These products can operate in a temperature range of 80 - 120 °C Purac PLA cups held boiling water and hot coffee without distortion compared to amorphous PLA cups [39]

The multilayer film of PLA coated with Si02 has good shielding ability, thus it is suitable for cheese packaging This film can keep the quality of cheese for 65 days [40] Polymer blends based on PLA, namely cellulose nanofibers and casein were very suitable for producing thermoformed packaging materials with tensile strength higher than the original PLA films [40-

41] Reis et al fabricated beeswax-coated trays based on PLA and thermoplastic starch by flat

extrusion techniques, rolling and heat processing The water-vapor permeability of

2%

PLA thermoplastic starch trays was significantly reduced due to the hydrophobic nature of the beeswax coating It is very suitable for preserving fresh fruits and vegetables [42],

4 2 Agriculture

PLA is used as a biological membrane to accelerate fruit ripening on crops, retain

fertilizers and moisture, inhibit the growth of mold, weeds and insect damage, and help plants cope with changes in weather Modified PLA is also used as nursery pots, pots and a number of alher items It has the ability to decompose into low molecular products, which gradually enter die soil after a certain period of use These decomposition products can become a source of notiients for plants [43],

V I V2 V3 V4

Figure 4 Behavior of tomato yams in green house trials VI: Romanian market - PP, V2: Incerplast -

PN C V3-V8: PLA composite fibers (UNIPIX) [44] (Reprinted from Maria et al with permission from

Romanian Biotechnological Letters)

Maria Rapa et al fabricated composite fibers based on PLA, wood fiber, and aliphatic-

aromatic copolyester (ECOFLEX F BX 7051) from 1,4-butandiol, adipic acid and terephtalic add monomers (UNIPIX) by melt extrusion method These fibers were applied as yams (a Aickoess of 0.3-0.5 mm, minimum resistance of 24 N) to support tomato plants during their powth in greenhouse trials (Figure 4) The authors showed that the tomato yam products V3, V4 V6, V8 and control VI have good quality and proper staking [44],

In recent years, FkuR Kunststoff GmbH Company has manufactured BIO-FLEX® in patkular from co-polyester and PLA Packaging films made from these blends have a high banner against moisture, oxygen and aromas with an adequate permeability (“breathability”) {45]-

4 J Textiles

Due to its small moisture absorption, low flammability, and UV resistance, PLA is very suitable for applications in the textile industry (yams, nonwovens, blankets, carpets, mattress, sportswear, sanitary products, diapers, etc.) [35, 46 - 51], Table 14 shows a comparison between

■Mure, recyclability and biodegradability of PLA fiber and PET fiber PLA was produced from